Mention how to add dragging for SpinBox's LineEdit

doc/classes/SpinBox.xml

@@ -25,6 +25,7 @@
 		The above code will create a [SpinBox], disable context menu on it and set the text alignment to right.
 		See [Range] class for more options over the [SpinBox].
 		[b]Note:[/b] [SpinBox] relies on an underlying [LineEdit] node. To theme a [SpinBox]'s background, add theme items for [LineEdit] and customize them.
+		[b]Note:[/b] If you want to implement drag and drop for the underlying [LineEdit], you can use [method Control.set_drag_forwarding] on the node returned by [method get_line_edit].
 	</description>
 	<tutorials>
 	</tutorials>